Overwhelm sister after one appointment (8)
I believe the answer is:
inundate
'overwhelm' is the definition.
(inundate can mean to overwhelm or overload)
'sister after one appointment' is the wordplay.
'sister' becomes 'nun' (sister is a term of address for a nun).
'after' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'appointment' becomes 'date' (synonyms).
'nun' put after 'i' is 'inun'.
'inun'+'date'='INUNDATE'
(Other definitions for inundate that I've seen before include "Flood - and unite (anag)" , "Flood, fill completely" , "be overwhelming" , "Flood, overwhelm" , "Overwhelm, swamp" .)